The presence of terminal transferase (TDT) enzyme is determined by an indirect fluorescent antibody assay on slide specimens from peripheral blood smears, bone marrow aspirate smears, cytocentrifuge preparations and frozen tissue sections. This approach is rapid, sensitive and utilizes a minimum of tissue material. The presence of terminal transferase is consistently demonstrated in normal thymus tissue, ALL at diagnosis and relapse, CML-Lymphoblast phase, and in Lymphoblastic lymphoma tissue. Normal lymphoid tissue, ALL in remission, AML, CML, other lymphomas and tumors as well as normal bone marrows do not demonstrate appreciable levels to TDT. Correlation of the immunoassay and enzyme activity assay is good. Currently immunohistochemical procedures are being evaluated to enhance the sensitivity and localization of TDT in tissues. Correlations of TDT activity with morphology, histochemistry, chemotherapy, and clinical course is also underway.